1. Field of the Invention
The present invention relates to electronic sensors and is concerned in particular with sensor fault error generation.
2. Discussion of the Background
With the integration of electronics within vehicle control, more and more electronic sensors are required that have a high signal integrity. For this reason, manufacturers have insisted on a minimum quiescent current flowing through all of the sensor connections, even the output pin. It is well known that with low voltage signal connections, a high quiescent current of several milliamperes improves connection reliability. To guarantee this current, the output pin will have either a pull-up or pull-down resistor connected within the monitoring ECU. This output loading resistor has the advantage that if the output connection at the sensor goes open circuit, the monitored signal will be pulled into an error band (ie a voltage level outside the normal sensor operating range), enabling the fault to be detected. In order to supply this increased output current while maintaining a wide voltage swing on the output, it is common for push-pull output stages to be used, consisting of two transistors. FIGS. 1a and 1b of the accompanying drawings show two typical configurations well known in this field.
The aforegoing load resistor, while solving the output open circuit fault, causes other failure detection problems. If, for example, either supply pin is open circuit, it is possible for enough current to flow back into the sensor, via the output pin, to produce an output signal within the normal operating range, therefore making the failure difficult to detect.